In the injection of fuel into an internal combustion engine it is necessary, because of emissions regulations and a desired low fuel consumption of the internal combustion engine, to determine precisely the quantity of fuel to be injected. The fuel quantity injected is determined by the injection duration and the fuel pressure. In common rail injection systems a plurality of injection valves is supplied with fuel at an adjustable pressure from a fuel reservoir. Through the opening of the injection valves during an injection process pressure fluctuations occur in the fuel reservoir, in the injection valve and in its feed line. The pressure fluctuations change the fuel pressure and must be taken into account in calculating the injection duration.
Known from EP 1 064 457 B1 is a method for determining the injection duration in an internal combustion engine operating with direct fuel injection, in which a total quantity of fuel that is to be supplied to a cylinder of the internal combustion engine is split into at least two injection processes per cylinder stroke. The pressure fluctuation arising through the preceding injection in the injection valve and in the line between the high-pressure reservoir and the injection valve is taken into account in determining the injection duration for the next injection process. To take account of the pressure fluctuations a correction term is determined, with which the pressure of the high-pressure reservoir is corrected. The injection duration is precisely determined using the corrected pressure value. The injection duration is calculated in dependence on the quantity of fuel to be injected and the corrected pressure value.